Field
The present invention relates to a method of manufacturing a membrane electrode assembly used for a fuel cell.
Related Art
A membrane electrode assembly (MEA) used for a fuel cell is a power generation element including an electrolyte membrane and electrodes (anode and cathode) formed on respective surfaces of the electrolyte membrane. Each of the electrodes includes an electrode catalyst layer that is placed to be in contact with the electrolyte membrane and a gas diffusion layer formed on the electrode catalyst layer. For example, JP 2013-182682A describes a method of manufacturing a membrane electrode assembly by sequentially stacking catalyst layers and gas diffusion layers on an electrolyte membrane.
A fluororesin (for example, Nafion (registered trademark)) that is a high-molecular polymer having a sulfonic acid group (—SO3H) as an end group is often used as an electrolyte material or more specifically an ionomer included in an electrode catalyst layer. The high-molecular polymer is likely to be deteriorated (decomposed) from its end group. Radial decomposition by the chemical action during power generation, as the measure factor, may cause decomposition of the sulfonic acid group of the ionomer in the electrode catalyst layer and thereby increase sulfate ion (SO42−). This may decrease pH in the fuel cell or more specifically in the membrane electrode assembly of the fuel cell to provide an acidic environment and cause poisoning of the electrode catalyst layer. Poisoning of the electrode catalyst layer may result in reducing the proton conductivity of the electrode catalyst layer and increasing the impedance of an electrode comprised of the electrode catalyst layer and a gas diffusion layer and may thus lead to reducing the power generation performance of the fuel cell. A radical scavenger (for example, cerium oxide) included in the gas diffusion layer of the membrane electrode assembly is used to suppress an increase of sulfate ion during power generation.
The inventors of the present application have found that the sulfonic acid group of the ionomer is decomposed by the action of the catalyst and the heat applied in the process of manufacturing the electrode catalyst layer or more specifically in its drying process to generate sulfate ion (SO42− and thereby cause poisoning of the electrode catalyst layer. This causes poisoning of an electrode catalyst layer even in an initial stage of a fuel cell (or more specifically membrane electrode assembly) and causes problems such as reduction of the proton conductivity of the electrode catalyst layer and increase in impedance of the electrode comprised of the electrode catalyst layer and the gas diffusion layer.
JP 2013-182682A fails to describe that sulfate ion generated in the manufacturing process of the electrode catalyst layer causes poisoning of the electrode catalyst layer even in the initial stage of the fuel cell (membrane electrode assembly) and causes problems such as reduction of the proton conductivity of the electrode catalyst layer and increase in impedance of the electrode comprised of the electrode catalyst layer and the gas diffusion layer.